ENCYCLOPEDIA

AGANIPPE (Aganippê), A nymph of the well of the same name at the foot of Mount Helicon, in Boeotia, which was considered sacred to the Muses, and believed to have the power of inspiring those who drank of it. The nymph is called a daughter of the river-god Permessus. (Paus. ix. 29. § 3; Virg. Eclog. x. 12.) The Muses are sometimes called Aganippides.

Source: Dictionary of Greek and Roman Biography and Mythology.

Pausanias, Description of Greece 9. 29. 5 (trans. Jones) (Greek travelogue C2nd A.D.) :
"On Helikon [the mountain of Boiotia], on the left as you go to the grove of the Mousai (Muses), is the spring Aganippe; they say that Aganippe was a daughter of the Termessos, which flows around Helikon."